Field of the Invention
The invention relates to an electrically heatable honeycomb body through which a fluid can flow, including wound, stacked or otherwise layered sheet-metal layers, in particular for use as a carrier body for a heatable catalytic converter in combination with an internal combustion engine, wherein electrical resistance varies an electrical current flowing through the carrier body. The invention also relates to a method for producing slits in the sheet-metal layers.
Heatable honeycomb bodies are employed as carrier bodies for catalytic converters for the exhaust gases of internal combustion engines and particularly as heatable catalysts or so-called precatalysts. In the catalytic conversion of combustion exhaust gases, it is important for the catalytic converter to have an operating temperature that is above an ignition temperature beyond which an exothermic catalytic conversion of the combustion exhaust gases begins. In the cold-starting phase of an internal combustion engine, the muffler and the catalytic converter of the exhaust system are not yet at a high enough temperature to provide sufficient catalytic conversion of the exhaust gases. As automobile traffic grows heavier and heavier, provoking ever more stringent exhaust gas regulations, it is a goal to assure virtually complete catalytic conversion of exhaust gases even during the cold-starting phase of engine operation. To that end, it is known to use electrically heatable catalytic converters, or heatable precatalysts that precede a conventional catalytic converter.
One such heatable electric honeycomb body is disclosed in International Patent Application WO 92/02714, corresponding to U.S. Pat. No. 5,411,711, for example. That reference in particular shows a kind of construction characterized by stacks of sheet-metal layers intertwined in an S. That lengthens the path of a current flowing through the electrically heatable honeycomb body.
Published International Application WO 92/13635, corresponding partially to U.S. patent application Ser. No. 08/353,964, filed Dec. 12, 1994, also discloses a honeycomb body that can be heated with nonhomogeneous current distribution, and thus with uneven heat output. That honeycomb body is formed of at least one, at least partially structured, metal sheet, which is wound or layered to make layers and is intertwined, forming a number of channels through which a fluid can flow. At least one electrical path is formed in the sheet-metal layers, and that path has an electrical resistance of nonhomogeneous distribution over the length of one such layer. The nonhomogeneity of the resistance is attained by slits, which are present at certain points or locations of the individual layers within the wound honeycomb body. Making slits in the individual layers before they are wound up to make the honeycomb body makes it possible to increase the electrical resistance of the honeycomb body as compared with one made up of unslit sheet-metal layers. Through the use of such slits, it is, for instance, possible for the electrical resistance of an electrically heatable precatalyst built into the exhaust system of a passenger car to be dimensioned in a purposeful way.
It is also known, from Published European Patent Application 0 316 596 A2, corresponding to U.S. Pat. No. 4,958,428, to make notches directly in the sheet-metal strips even before the carrier body is wound or folded up. Carrier bodies made in that way are capable of dissipating thermal strains during operation, between the jacket tube and the sheet-metal strips wound up in it.
For the sake of a purposeful configuration of the electrical resistance of a heatable carrier body, as in the prior art cited, purposeful slits or slit structures must be machined into the individual sheet-metal layers. However, in the ensuing winding, stacking or otherwise layering of the sheet-metal layers, such slits in a sheet-metal layer often become bridged over by raised points in an adjacent sheet-metal layer, which leads to undesirable, poorly replicable changes in the electrical resistance of such a carrier body. Moreover, such spotwise "short circuits" can cause disproportionately high current densities in those regions, which can lead to very hot spots. Such an over-bridging phenomenon is especially serious in the case of slits which are made obliquely in the sheet-metal layers relative to the structures thereof.
In order to construct carrier bodies with a predetermined electrical resistance, it is initially important for the resistance of the individual sheet-metal layers to be determined precisely, by measurement and calculation, before the sheet-metal layers are processed to make a carrier body. Subsequently, for a predetermined type of stacking, winding or the like of the individual sheet-metal layers, the expected electrical resistance of the overall carrier body can be accurately predetermined. However, that is only possible if no uncontrollable bridging over of slits, with the attendant short circuits, occurs within the resultant carrier body when the individual sheet-metal layers are processed to make a carrier body.